engine suddenly locked/jammed

jaycoop

Member
Hello, Auction purchased a c.1950 MM ZAU, hasn't run in maybe 15 years (sat outside). Pulled plugs and soaked a bit. Motor spun freely, several times as I checked compression (pretty good) and valve action (all operate). Went to spin it again and she won't budge (starter works well, just clunks into the ring gear). Ring gear is good, no missing teeth. I placed a wrench on the crank nut - doesn't budge, locked tight. Note that nothing else has been apart and nothing has fallen into any cavities. What would suddenly prevent rotation of the motor, when just prior to this it spun freely?
 
Take the clutch cover off and use a bar to rotate it in reverse. Pull the plugs and blow out the cylinders with the pistons near TDC.

You may have carbon falling loose from combustion chamber and or valves.
 
I would take the head off and look inside. Probably some debris fell through a valve into the cylinder and jammed it.
Don't get too rough with it until you find the problem.
 
Thanks much for the additional help and ideas. I pulled the starter and tried turning the engine again in both directions - nothing. I will look into the matter of the head/dislodged debris. To me, it is such solid resistance that I wonder if there isn't something metal in the way. Is it possible that the roller bearing at the front of the engine has fallen apart, causing the jam. I've read elsewhere of this problem with this particular motor, although no one has ever indicated that the bearing completely fell apart....rather they are notorious for failing/becoming noisy/requiring replacement. Also, what about the oil pump or some other device that moves mechanically with the engine?
 
With starter off, in neutral, clutch disengaged: try putting a bar into starter hole and into flywheel teeth. Try moving it both directions. If it is stuck tight, may need to check for spun rod or main bearing.
 
I was working on an R years ago, it would turn over just fine but as we were rolling it over to get it timed it got harder an harder at one spot until it finally locked up. i got it to turn backwards and started tearing into it.It turned out some mice had got up into the engine and filled a port with millet, every time I would turn it over a valve would open and dump millet into the cylinder until it got enough to keep it from going to top dead center.
 
That is the best ever possible cause! I've been there and I do think something odd is going on here. Many years ago my dearly departed brother and I were starting a Buda engine in a Huber rowcrop tractor and nuts packed away in the manifold by squirrels came rapid firing out at him as he controlled the carburetor and I hand cranked the motor.
 

Found anything yet??

We?ve also run into mice storing away milo/corn in an engine.

Also, it?s probably a long shot, but I had a mag on a 605 irrigation engine lock down tight enough to shell half the teeth off the cam gear!!!

In other words, sometimes it pays to check other than the main engine itself.

I?d also throw a vote towards getting a borescope. They aren?t too expensive these days. The one I got connects via WiFi to my cell phone and lets me see the pic on the screen and/or take screen captures for reference. Snaking a camera down a plug hole has really helped me a lot on things like that.

Howard
 
Many thanks to all who have helped try to figure this out, esp. jdacih! Hard to believe it could happen, until I pulled the head and manifolds today and got a good look inside. Sure enough there was a good amount of small seeds and assorted plant matter down in the ports, some of which managed its way onto the top of the pistons, and combined with the carbon build-up that was already there, when 2 and 3 came up to top dead center, it came to a sudden stop. I couldn't get a good enough hold on the flywheel to turn it backward, so I removed the side cover (which I wanted to do anyway, to inspect things) and put a bar between the crankshaft and camshaft and gently pried in reverse and she freed right up. Then with scraping, vacuuming and compressed air, I got things cleaned up real good, including all ports, passages, combustion chamber, and piston tops. I can see that the cylinder walls are nice and clean and not scored, and not much of a ridge....also, looking into the bowels of the motor, I can see that it is quite clean and undoubtedly was rebuilt some time ago, as the job of putting the cotter pins back in was not up to factory specs! I have a feeling it will run, but not counting my eggs yet. Thanks again!!
 
Another tip related to this, hoping someone will benefit from it. Do not assume the encrusted debris will be only on the top of the pistons. I used a bright light and an inspection mirror, to discover that there were hardened deposits on the underside (blind end) of the bores, especially right near the outer edges - at first you don't see it. It's enough to bring the engine to a dead stop when cranking. I heated and bent a couple of different sharpened screwdrivers and narrow scrapers to remove the build-up (straight tools will not reach it). I believe this is what caused the former owner to park the tractor 15-20 years ago. Even an engine that has been rebuilt could have this problem, depending on the quality of the rebuild/knowledge of the rebuilder. You can't see it unless you are looking closely and carefully.
 

Im glad you posted this! Im lookign at a 602 that hasnt been started in about 30 yrs. you care to say how much it was? Im trying to guage what a fair price is lol
 
I paid 500.00, but of course the 602 is a quite different machine and I am no expert on the values. For 500.00, I got a tractor that was complete. What it needed to get it going: clean out/hone piston bores, new rings, engine gasket set, carb rebuild, water pump rebuild, one used rear wheel, replaced wiring harness and gauges (I made up the wires), freed up the brakes. I'm not restoring it, just making it a usable tractor. I did all the work myself, so a low budget project.
 

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